Natural gas wells typically produce a mixture of natural gas, hydrocarbon condensate and water. There are a number of points in the production process where high pressure drops occur, resulting in a corresponding temperature drop caused by thermal expansion (Charles' Law). Freezing of the water and/or hydrocarbon condensate occurs when the thermal expansion temperature drop is coupled with low ambient temperatures, causing disruption of the well production. Furthermore, crude oil can be a liquid with relatively high viscosity that is inversely proportional to temperature. As ambient temperature drops below ˜50° F., the viscosity of produced crude oil can rise to a point where the flow properties of the crude become problematical. To combat these problems producers originally brought in line heaters for the winter season. Because of safety concerns, producers subsequently switched to pumps set up in the winter season to inject methanol into the production streams, typically gas-powered pneumatic injection pumps.
A typical methanol practice today for remote and largely unmanned production locations is to turn on a pneumatic methanol injection pump at the onset of a “winter season.” The pump runs continuously during the defined “winter season.” The “winter season” is defined by experience at the well, selecting an initial date when ambient temperature may first be expected to dip below a selected temperature limit and selecting a terminal date when experience indicates that the temperature will not drop below the limit until the next season. The dates for a “winter season” will be a function of the geographic location of the remote facility and factors at the production location. Reliably defining the “winter season” is a duty for experienced operators in the field.
The temperature limit for defining the “winter season” is selected depending upon the well, the production stream and possibly a variety of other factors, in order to avoid freezing and/or poor flow. Such selected ambient temperature limit typically occurs within the range of 40° F. to 60° F.
Conservative temperature limits are usually selected for defining the onset of the “winter season” and for defining the end of the “winter season” because errors are costly. Because errors are costly, methanol injection in remote oil and gas production locations demands a high level of reliability. The cost for a day's lost production of natural gas, due to nonflow in a line, may be estimated to average about $500 per day.
Pneumatic chemical injection pumps are relatively inexpensive and historically favored, typically running around $500 to $700. Pneumatic pumps have a well established track record for high reliability. For low cost and high reliability reasons, remotely located methanol injection pumps have been traditionally powered pneumatically, the pneumatic motive force being reliably provided by the gas or fluid flowing through the production stream.
Environmental influences led to the development of solar rechargeable battery powered chemical injection pumps for certain chemical operations in remote locations, as “solar pumps” do not vent gas. However, solar pumps are expensive, running about 4 to 5 times the cost of a pneumatic pump, or around $2,500, and have reliability issues. There is doubt as to whether solar power would be able to provide an adequately charged battery 100% of the time.
As a result of reliability issues and high cost, solar-powered injection pumps have not enjoyed large use for methanol injection applications. Methanol injection requires high reliability. The very day a solar powered injection pump is most likely to be inoperative, the short, lowlight, cloudy winter day, coincides with the day that methanol injection is likely to be the most important. Although additional batteries could be added to a solar pump, they would almost double the cost again.
The instant invention arose based upon a surprising discovery: a surprisingly high, an unexpectedly high, amount of methanol, and thus money, is wasted by leaving a methanol injection pump running continuously “on” for a “winter season.” This fact appears true over more or less the full range of particular temperature limits that might be selected to define a “winter season.” A study by the instant inventors documented this surprising cost of the unnecessary expenditure of methanol. The extent of the loss had not been disclosed or documented by the industry prior to the inventor's study.